Biaxially oriented polyester films are used in various applications because of their excellent thermal properties, dimensional stability and mechanical strengths and easy controllability of their surface morphology. It is especially well known that they are useful as substrates of magnetic recording media, etc. In recent years, magnetic recording media such as magnetic tapes are required to be higher in density for adaptation to the machines and materials which become lighter in weight, smaller in size and larger in capacity. For recording at higher densities, it is useful to make the recording wavelength shorter and the recording track smaller. However, if the recording track is made smaller, there arises a problem that the recording track is liable to deviate because of the deformation caused by the heat during tape running or by the changes of temperature and humidity during tape storage. Therefore, the demand for such properties as dimensional stability of tapes in service environment and storage environment grows stronger.
From this point of view, highly stiff aromatic polyamides more excellent in strength and dimensional stability than biaxially oriented polyester films are sometimes used for substrates. However, aromatic polyamides are very expensive and costly and cannot be realistic substrates for general purpose recording media.
On the other hand, magnetic recording medium substrates using polyester films of polyethylene terephthalate, polyethylene naphthalate, etc. enhanced in strength by means of stretching technology are developed. However, they cannot yet satisfy the severe requirement for dimensional stability under changing temperature and humidity conditions.
Further, for improving the dimensional stability under changing temperature and humidity conditions, a method of forming a reinforcing layer of a metal or the like on one surface or each of both the surfaces of a polyester film (Patent Document 1) is disclosed. However, in the case where the reinforcing layer is formed of a metal, the layer is highly conductive and has a nature of reflecting light because of metallic bonding. For this reason, though transmission light is used to control the film thickness of the magnetic layer formed by coating, there arises a problem that the reinforcing film formed of a metal does not allow the transmission of light. So, film thickness control becomes difficult, and since the magnetic layer becomes irregular in thickness, a magnetic tape with a large error rate is liable to be formed. Further, since the magnetic tape has high conductivity, static electricity and leak current cause a current to flow in the magnetic tape, and the current may short-circuit or trouble the magnetic head. Furthermore, there is another problem that since a metal is lower in strength than an oxide, the effect of inhibiting the expansion and contraction of the polyester film is small. On the other hand, in the case where the reinforcing layer is formed of an oxide or any other compound, it has such natures as being hard but fragile and not ductile because of ionic bonding. So, tension may cause cracking, and curving may cause cracking. Moreover, since an oxide is hygroscopic, the reinforcing layer is small in the effect of improving dimensional stability under a changing humidity condition, and the hygroscopic expansion of the reinforcing layer per se may lower the dimensional stability.
The inventors made an intensive study and as a result, found that if the oxidation degree of the reinforcing layer is controlled instead of perfectly oxidizing the metal, the dimensional stability can improve dramatically, and many of the above-mentioned problems can be solved.
Meanwhile, a technique of vapor-depositing a metal oxide layer controlled in oxidation degree is disclosed as a gas barrier film (Patent Document 2). However, the film described in this document is a packaging film intended to be used as a gas barrier, and since it is required to be transparent, the vapor deposition film has a small thickness of 40 nm or less, and the metal oxide layer is small in the effect of inhibiting the expansion and contraction of the polyester film. Further, to vapor-deposit a 50 nm or thicker metal oxide layer controlled in oxidation degree, it is necessary to increase the evaporated amount of aluminum, and accordingly, it is necessary to increase also the introduced amount of oxygen. However, according to the method described in this document, since a vacuum evaporator as shown in FIG. 3 is used, it is difficult to increase the thickness of the metal oxide layer. That is, in the vacuum evaporator 111, a polyester film runs from an unwinding roll 113 along a cooling drum 116 to a winding roll portion 118 in a vacuum chamber 112. At this time, a metallic material 119 in a crucible 123 is heated and evaporated by an electron beam 121 emitted from an electron gun 120, and at the same time, oxygen gas is introduced from oxygen supply nozzles 124 to perform an oxidation reaction with the evaporated metal, the reaction product being vapor-deposited on the polyester film on the cooling drum 116. However, since the oxygen supply nozzles 124 are installed near the cooling drum 116, the increase in the introduced amount of oxygen causes the oxygen gas flow to scatter the metal vapor, making it difficult to control the oxidation degree. Further, partly because the space where the metal and oxygen react with each other is small, it is difficult to form a 50 nm or thicker metal oxide vapor deposition layer, and the formed vapor deposition layer is liable to be unstable. The unstable vapor deposition layer produces numerous structural defects, to lower the dimensional stability. Moreover, since the gas barrier film is used as a packaging material, the base film is as thick as 10 μm or more and is not smooth on the surface, to allow easy vapor deposition. On the contrary, the polyester film used as a magnetic recording medium substrate is generally thin in thickness and smooth, and if vapor deposition is performed without any particular contrivance by a method as described above, heat deformation or the like can cause the film to be broken frequently during vapor deposition.    [Patent document 1] JP7-272247A    [Patent document 2] JP62-220330A